Re: [PATCH bpf-next v3 4/8] selftests/bpf: make arena_base() opaque to GCC's -Warray-bounds

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Mon, Nov 02, 2026 at 02:07 PM Vineet Gupta <vineet.gupta@linux.dev> wrote:
> The barrier is behind #ifdef __clang__ so clang keeps the plain macro.
> It does not need it, and the barrier is not free: it blocks reuse of
> the map address, so each call site reloads it.  Across the five progs
> and 20 call sites that is +35 instructions under GCC
> (2528 -> 2563), about 1.4% on those objects and nothing elsewhere.
> Clang output is unchanged, bit for bit.

stream.c already has barrier_var() for both compilers.

How about this one instead:

diff --git a/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h b/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
index 82aafe8..0eba529 100644
--- a/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
+++ b/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
@@ -49,7 +49,16 @@ void __arena* bpf_arena_alloc_pages(void *map, void __arena *addr, __u32 page_cn
 int bpf_arena_reserve_pages(void *map, void __arena *addr, __u32 page_cnt) __ksym __weak;
 void bpf_arena_free_pages(void *map, void __arena *ptr, __u32 page_cnt) __ksym __weak;

-#define arena_base(map) ((void __arena *)((struct bpf_arena *)(map))->user_vm_start)
+/*
+ * The map definition is smaller than struct bpf_arena.
+ * barrier_var() hides that from -Warray-bounds of GCC.
+ */
+#define arena_base(map)							\
+	({								\
+		struct bpf_arena *___arena = (void *)(map);		\
+		barrier_var(___arena);					\
+		(void __arena *)___arena->user_vm_start;		\
+	})

 #else /* when compiled as user space code */

diff --git a/tools/testing/selftests/bpf/progs/stream.c b/tools/testing/selftests/bpf/progs/stream.c
index 1ae015c..b4d88d0 100644
--- a/tools/testing/selftests/bpf/progs/stream.c
+++ b/tools/testing/selftests/bpf/progs/stream.c
@@ -178,16 +178,8 @@ __stderr("Call trace:\n"
 "|[ \t]+[^\n]+\n)*}}")
 int stream_arena_write_fault(void *ctx)
 {
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start;
+	u64 user_vm_start = (u64)arena_base(&arena);

-	/* Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than struct
-	 * bpf_arena. barrier_var() makes the pointer opaque to GCC, preventing the
-	 * bounds analysis
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	asm volatile (
@@ -212,16 +204,8 @@ __stderr("Call trace:\n"
 "|[ \t]+[^\n]+\n)*}}")
 int stream_arena_read_fault(void *ctx)
 {
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start;
+	u64 user_vm_start = (u64)arena_base(&arena);

-	/* Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than struct
-	 * bpf_arena. barrier_var() makes the pointer opaque to GCC, preventing the
-	 * bounds analysis
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	asm volatile (
@@ -252,17 +236,9 @@ int stream_arena_load_acquire_fault(void *ctx)
 		.off	 = 0x7fff,
 		.imm	 = 0x100,	/* BPF_LOAD_ACQ */
 	};
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start, val;
+	u64 user_vm_start = (u64)arena_base(&arena);
+	u64 val;

-	/*
-	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than
-	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
-	 * preventing the bounds analysis.
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	asm volatile (
@@ -296,17 +272,9 @@ int stream_arena_xchg_fault(void *ctx)
 		.off	 = 0x7fff,
 		.imm	 = 0xe1,	/* BPF_XCHG */
 	};
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start, val;
+	u64 user_vm_start = (u64)arena_base(&arena);
+	u64 val;

-	/*
-	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than
-	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
-	 * preventing the bounds analysis.
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	/*
@@ -346,17 +314,9 @@ int stream_arena_cmpxchg_fault(void *ctx)
 		.off	 = 0x7fff,
 		.imm	 = 0xf1,	/* BPF_CMPXCHG */
 	};
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start, val;
+	u64 user_vm_start = (u64)arena_base(&arena);
+	u64 val;

-	/*
-	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than
-	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
-	 * preventing the bounds analysis.
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	/*